The invention generally relates to methods and apparatus for the electronic reproduction of an image. More specifically, the invention is directed to a method and to an apparatus for the analysis of color casts in color originals in apparatus and systems for electronic image processing. As is known, a color cast analysis of color originals is undertaken to acquire setting parameters for color cast correction of the color originals.
As used herein, the term "image original" refers to original images whether black and white or color. "Color original" refers to original images in color, e.g., color photographs.
As is known, electronic image processing essentially comprises the steps of image input, image processing and image output.
In the image input step utilizing, for example, a color image scanner (scanner) as an image input device, three primary color value signals R, G, B can be acquired by trichromatic as well as pixel-by-pixel and line-by-line scanning of color originals to be reproduced with an optoelectronic scanner element, whereby the individual color value triads represent the color components "red" (R), "green" (G) and "blue" (B) of the picture elements scanned in the color original. The analog color values are converted into digital color values and are stored for the subsequent image processing.
In the image processing step, the color values R, G, B are usually first converted into color separation values C, M, Y, K according to the regularities of subtractive color mixing, these color separation values being a measure for the dosages of the inks "cyan" (C), "magenta" (M), "yellow" (Y) and "black" (K) or, respectively, for the raster point sizes or raster percentages employed in a later printing process.
Over and above this, various image parameters such as light image values and dark image values for an adaptation of the image scope, color cast values for a correction of color cast as well as a corrected image gradation characteristic for a contrast correction or for a correction of over-exposures and under-exposures can be set. Further, local and selective color corrections can also be undertaken, with the object of improving the image reproduction, compensating deficiencies or undertaking editorial changes.
The setting of the image parameters by an operator usually begins with the operator first presetting standard values that he calculates based on a rough preclassification of the appertaining color original or based on an experienced guess. While setting the image parameters, the operator makes use of the measuring functions of the color image scanner in that he measures characteristic picture elements in the color original with the optoelectronic scanner element with respect to image scope, color cast and luminance distribution and employs the measured results for calculating optimum setting values.
After the image processing, the image output is produced with a suitable image output device, for example with a color separation recorder or printer for the rastered recording of color separations on a film material.
A color cast analysis in a color original and a color cast correction of the color original based on the analysis are required because the color original (usually a photographic reproduction of an original image) frequently has chromatic deviations when compared to the actual original image, these usually being exposure-conditioned and film-oriented errors. Further, with respect to color originals scanned with a color image scanner with basic settings, at least the coloring of the film carrier material is present in the resulting color values as an undesired color cast.
For correcting color cast, the operator must find the correct white point and black point in the color original by visual evaluation and must identify the color cast values on the basis of a color measurement. The interpretation of the measured results and their conversion into optimum setting values for the color cast correction require a great detail of experience and often represent difficulties for an unexperienced operator.
When chromatic errors of a color original are to be identified without the color sensation of an operator, an automatic color cast analysis must be implemented wherein the color information of the color original is evaluated and a decision is made based on specific, objective criteria as to whether there is a color cast or not. Since an automatic color cast analysis cannot provide any information about the theme of the image or, respectively, about how the image should actually look, statistical statements must be utilized as the criteria.
It is already known to undertake automatic color cast analyses of color originals by evaluating the color values of the color originals and to employ the results of the analysis for calculating image-dependent, pre-setting values for the color cast correction. The operator can evaluate the result of a color cast analysis and directly transfer the resulting pre-setting values into the image input device or can modify or, respectively, correct these on the basis of measurement functions in order to undertake an optimum setting. The operator is thus relieved of routine jobs or tasks and can concentrate on the processing of color originals wherein additional global or selective color corrections are required for improving the reproduction quality.
The known methods for the analysis of a color cast in a color original have the particular disadvantage that they do not allow any reliable identification of the optimum setting values for an optimally broad spectrum of color originals, so that no simple, fast and standardized parameterizations of image input devices are possible.
The known methods for color cast analysis are based on the color values R, G, B of a device-dependent RGB color space acquired by the respective image input device, whereby the analysis of the image scope and of the color cast is directly implemented with reference to the color values R, G, B, and whereas a luminance signal derived from the color values R, G, B is often employed for analysis of the image gradation.
A further disadvantage arises in that the known analysis methods must be specifically adapted to the properties of the color values R, G, B of the respective image input devices given the use or connection of different image input devices.
The known analysis methods, moreover, are calculation-intensive since the color values R, G, B acquired with the image input devices must first be resolved into color components for the analysis of a color cast and must also be additionally resolved into a luminance component for the analysis of image scope and image gradation.